Magnetic Field

Nov. 23, 2018: When a stream of solar wind hits Earth, magnetometers around the Arctic Circle normally go haywire, their needles swinging chaotically as local magnetic fields react to the buffeting of the solar wind. On Nov. 18th, however, something quite different happened. Solar wind hit Earth and produced … a pure, almost-musical sine wave:

Rob Stammes recorded the event from the Polarlightcenter, a magnetic observatory in the Lofoten Islands of Norway. “A very stable ~15 second magnetic oscillation commenced and persisted for several hours,” he says. “The magnetic field was swinging back and forth by 0.06 degrees, peak to peak, with the regularity of a metronome.”

Imagine blowing across a piece of paper, making it flutter with your breath. The solar wind can have a similar effect on magnetic fields. The waves Stammes recorded are essentially flutters propagating down the flanks of Earth’s magnetosphere excited by the breath of the sun. Researchers call them “pulsations continuous” — or “Pc” for short.

“A very sensitive magnetometer is required to record these delicate waves,” says Stammes. “I use a mechanical magnetometer with bar magnets suspended from a special wire. LEDs and light detectors in an isolated dark box record the motion of the magnets, while vanes in oil damp out non-magnetic interference.”

Pc waves are classified into 5 types depending on their period. The waves Stammes recorded fall into the range 15 to 45 seconds–that is, Pc3. Researchers have found that Pc3 waves sometimes flow around Earth’s magnetic field and cause a “tearing instability” in our planet’s magnetic tail. This, in turn, can set the stage for an explosion as magnetic fields in the tail reconnect.

A quartet of NASA spacecraft recently flew through just such an explosion. Last week, researchers from the University of New Hampshire reported that four Magnetospheric Multiscale (MMS) spacecraft spent several seconds inside a magnetic reconnection event as they were orbiting through Earth’s magnetic tail. Sensors on the spacecraft recorded jets of high energy particles emerging from the blast site. One jet was aimed squarely at Earth and probably sparked auroras when it hit the upper atmosphere.

Stammes has recorded many Pc waves in the past, “but this is the first time I have detected category Pc3,” he says. “This was a very rare episode indeed.”

June 10, 2018: For the past two years, Spaceweather.com and the students of Earth to Sky Calculus have been traveling around the world, launching cosmic ray balloons to map our planet’s radiation environment. Our sensors travel from ground level to the stratosphere and bring their data back to Earth by parachute. Here is a plot showing radiation vs. altitude in Norway, Chile, Mexico, and selected locations in the USA:

Note: Data from Sweden and several other US states are omitted for the clarity of the plot.

We’re about to add a new country to the list: New Zealand. On June 18th, a team of students from Earth to Sky is traveling to New Zealand’s north island to launch 3 cosmic ray balloons in only 10 days. Soon, we will know more about cosmic rays above Earth’s 8th continent.

Cosmic rays are, essentially, the subatomic debris of dying stars, accelerated to nearly light speed by supernova explosions. They travel across space and approach Earth from all directions, peppering our planet 24/7. When cosmic rays crash into Earth’s atmosphere, they produce a spray of secondary particles and photons that is most intense at the entrance to the stratosphere. This secondary spray is what we measure.

The purpose of our mapping project is to study how well Earth’s atmosphere and magnetic field protects us from cosmic rays. As the plot shows, the shielding is uneven. More radiation gets through to the poles (e.g., Norway) and less radiation penetrates near the equator (e.g., Mexico).

But there’s more to the story. Our launch sites in Chile and California are equidistant from the equator, yet their radiation profiles are sharply different. Chile is on the verge of the South Atlantic Anomaly, which almost surely distorts the radiation field there. Our flights over New Zealand may shed some light on this, because our launch sites in New Zealand will be the same distance from the equator as the sites in Chile. Stay tuned!

March 11. 2018: The vernal equinox is less than 10 days away. That means one thing: Cracks are opening in Earth’s magnetic field. Researchers have long known that during weeks around equinoxes fissures form in Earth’s magnetosphere. Solar wind can pour through the gaps to fuel bright displays of Arctic lights. One such episode occurred on March 9th. “The sky exploded with auroras,” reports Kristin Berg, who sends this picture from Tromsø, Norway:

During the display, a stream of solar wind was barely grazing Earth’s magnetic field. At this time of year, that’s all it takes. Even a gentle gust of solar wind can breach our planet’s magnetic defenses.

This is called the the “Russell-McPherron effect,” named after the researchers who first explained it. The cracks are opened by the solar wind itself. South-pointing magnetic fields inside the solar wind oppose Earth’s north-pointing magnetic field. The two, N vs. S, partially cancel one another, weakening our planet’s magnetic defenses. This cancellation can happen at any time of year, but it happens with greatest effect around the equinoxes. Indeed, a 75-year study shows that March is the most geomagnetically active month of the year, followed closely by September-October–a direct result of “equinox cracks.”

NASA and European spacecraft have been detecting these cracks for years. Small ones are about the size of California, and many are wider than the entire planet. While the cracks are open, magnetic fields on Earth are connected to those on the sun. Theoretically, it would be possible to pick a magnetic field line on terra firma and follow it all the way back to the solar surface. There’s no danger to people on Earth, however, because our atmosphere protects us, intercepting the rain of particles. The afterglow of this shielding action is called the “aurora borealis.”

Feb. 22, 2018: Today, a high speed solar wind stream is passing just south of Earth, making grazing contact with our planet’s magnetic field. This is causing something unusual to happen. Around the poles, Earth’s magnetic field has been ringing like a bell. Rob Stammes recorded the phenomenon from his magnetic observatory in Lofoton, Norway.

“Ths morning, the magnetic field around our observatory (as measured by ground currents) was swinging back an forth with a 100 second period,” says Stammes. “This very stable oscillation went on for more than an hour.”

This is quite different from what normally happens when a solar wind stream hits Earth’s magnetic field. Here is an example of Stammes’ recordings during a regular geomagnetic storm. Compared to the cacophany of a normal storm, this morning’s event was a sweet pure tone.

Researchers call these pure ultra-low frequency oscillations “pulsations continuous” (Pc). Pc waves have an energizing influence on particles in Earth’s inner magnetosphere because they resonate with the natural motion of particles around the geomagnetic field. This, in turn, can supercharge the aurora borealis.

Some of the energy injected by Pc waves is being observed right now in Sweden. “The auroras are going crazy!” reports Chad Blakley of Lights over Lapland, who roared out on his snowmobile to photograph the display:

“The lights were so impressive that I forgot that I was only wearing jeans before heading out! It may have been -25 degrees outside but it was worth 15 minutes in the cold to see a display that I will never forget,” says Blakley.

The effect of this solar wind stream may be likened to a person blowing across the top of a soda bottle, the glancing breath producing a nearly monochromatic waveform. “This is quite rare,” says Stammes. “Pulsating continuous signals like these are visible only 2 or 3 times a year.”

Stay tuned for more “ringing auroras” in the hours ahead. Free:Aurora Alerts